Shells

History-dependent functions, such as the history command, and command editing using vi and emacs will not work.

Active job names will be unknown on the jobs command.

Some scripts might not work as expected.

Replaces Bourne shell:

Supports 4-byte EUC.

Now conforms to XPG4 specifications.

Three new or changed commands: hash, type, export.

Position of reserved file descriptors changed.

"hash" becomes a shell built-in command with the addition of -r.

New built-in command type.

export -p will be fully implemented.

Positional parameters.

When a function call returns, the POSIX shell restores the positional parameter list. This does not happen in the Bourne shell, where the positional parameters are global to the functions.

For example, if a shift operation is performed within a function invoked by the POSIX shell, its effect will not be felt after the function returns.

Errors in functions.

These abort the function but not the script.

Lookup order.

Function lookup is done before the built-in commands.

Error messages.

Some error messages output by the POSIX shell are different from Bourne shell messages.

LC_TYPE, LANG

Changing the value of LC_CTYPE or LANG after the shell has started will not affect the lexical processing of shell commands in the current shell execution environment or its subshells.

IFS

The shell treats each character of IFS as a delimiter.

select

This command does not exist in the Bourne shell.

New built-in commands.

Examples: whence, alias, fg, bg, jobs, fc, getopts, kill, let, unalias.

read, print

These have a -p option for co-processes.

New operators.

Examples:

 

   [[..]], ((...)), >|, and |&

!

Any command preceded by a "!" returns the logical negation of the exit status of the command.

Reserved words.

Examples:

   ! 

   selectd

   function 

   [[

   ]]

Aliases.

Aliases can be used to re-define special built-in commands. Aliases can be created, listed and exported.

New parameter substitutions.

In addition to the substitutions allowed in the Bourne shell, substitutions can be performed with the following:

   ${name#pattern}

   ${name##pattern}

   ${name%pattern} 

   ${name%%pattern}

One-dimensional array facility.

An element of an array is referenced by a subscript. set -A can be used to assign values to an array.

Tilde substitution.

Tilde substitution can be performed.

Parameters.

In addition to parameters such as #, -, ?, $ that are set automatically in the Bourne shell, the following parameters are set by the POSIX shell: ERRNO, LINENO, OLDPWD, OPTIND, OPTERR,OPTARG, PWD, PPID, REPLY, RANDOM SECONDS.

Similarly, in addition to the parameters such as IFS and CDPATH that are used by the Bourne shell, the following parameters are used by the POSIX shell: EDITOR, ENV, FCEDIT, VISUAL, PS3, PS4, HISTFILE, HISTSIZE.

Positional parameters.

Positional parameters are not limited to 9 as in the Bourne shell.

Integer arithmetic.

Integer arithmetic can be performed using the let command. Or you can use the following construct:

   ((...))

New input/output operators.

These include:

   >|, <>, <&p, and >&p

Single-step assignment.

Values can be assigned in a single step using readonly and export (for example export LANG=french) instead of the two steps needed in the Bourne shell.

Symbolic names.

Symbolic names for signals and traps can be used.

Environment variables automatically inherited.

An environment variable is passed to the child process even if it is not exported.

for, while, until

for, while, and until loops are executed in the current process environment. Assignments made within loops remain effective even after the loop completes.

Job control.

Job control capabilities allow the user to suspend, resume, and control processes executing in the background.

Command-line editing.

You can do command-line editing with vi or emacs editing commands.

History.

You can use a history file to list and edit command lines.

! command(s)

Negates the exit status of the command(s) just executed.

command [arg ...]

Treats arg as a command, but disables function lookup on arg.

export -p

Writes to the standard output the names and values of all exported variables, using quotes so that they can be re-input to the shell as commands that export the same variables in the same way.

getopts optstring name [ arg ... ]

name contains ? when end of options is found.

hash [external commands/programs] or hash -r

The hash command adds external command locations to its list of remembered locations. When no arguments are specified, it reports on the contents of the list.

The -r option removes the locations of all previously remembered external commands.

kill -l

Lists signal names.

kill -L

Lists signal numbers and names. Produces the same output as kill -l in the Korn shell.

readonly -p

Writes to the standard output the names and values of all read-only variables, using quotes so that they can be re-input to the shell to set the same attributes to the same values.

set +

Lists the names of all shell variables without their values.

set -C

"Noclobber." Can also be set (with -C) when the shell is invoked.

test -e file or

   [[ -e <file> ]]

True if file exists.

type

Indicates how each argument would be interpreted if used as a command name.

umask -S

Prints the current value of the mask in symbolic format.

unalias -a

Removes all the alias definitions from the current shell execution environment.

unset -v names

"Unassigns" names (erases their values and attributes). If the -v option is used, names refer to variable names.

Unsetting ERRNO, LINENO, MAILCHECK, OPTARG, OPTIND, RANDOM, SECONDS, TMOUT and _ removes their special meaning even if they are subsequently assigned to.

whence -p name

Output is different from ksh.

The -p option does a path search for name even if name is an alias, a function, or a reserved word.

IFS

See “Differences between the Bourne and POSIX Shells” earlier in this chapter for a description of IFS in sh-posix.

OPTERR

New sh-posix parameter.

Lookup order:

-c

If sh-posix is invoked with -c, a process will be spawned for it:

   $ /usr/bin/sh -c "ps -f"

   UID   PID  PPID   C STIME    TTY      TIME COMMAND

   usera 511  510    3 16:15:46 ttyp8    0:00 ps -f

   usera 501  500    0 16:15:36 ttyp8    0:00 -ksh

   usera 510  501    1 16:15:45 ttyp8    0:00 /usr/bin/sh -c ps -f

Signals

sh-posix has the following signals:

     0) NULL 

     6) ABRT    (IOT for ksh)

Tilde expansion

sh-posix provides tilde expansion on the fly for the following parameter substitutions:

    ${parameter:-word}

    ${parameter:+word}

    ${parameter:=word}

    ${parameter:?word}

For example, if ~ is /users/usera:

   $ a=${strings:-~}



   a --> /users/usera (sh-posix)

   a --> ~            (ksh)

Exit values for failed exec.

sh-posix exits with:

Positional parameter 0

When executing a function, sh-posix leaves positional parameter 0 unchanged. ksh sets this parameter to the function name.

Keyword time

Does not exist in sh-posix.

_

Is not a special variable in sh-posix, so no special processing is done.